1. Technical Field
The present disclosure relates to capacitive position sensing in an electrostatic micromotor, in particular for atomic-level storage systems generally known as “probe storage” systems.
2. Description of the Related Art
As is known, storage systems exploiting a technology based on magnetism, such as, for example, hard disks, suffer from important limitations regarding the increase in the data-storage capacity and the read/write rate, and the reduction in their dimensions. In particular, there is a physical limit, the so-called “superparamagnetic limit”, which constitutes an obstacle to the reduction in the dimensions of the magnetic-storage domains below a critical threshold, if the risk of losing the information stored is to be avoided.
Consequently, in the last few years, alternative storage systems have been proposed, amongst which the so-called “probe storage” systems have assumed particular importance. These systems enable high data-storage capacities to be achieved with reduced dimensions and with low manufacturing costs.
As illustrated in FIG. 1, a storage device 1 of the probe-storage type comprises an array 2 of transducers (or probes) 3, arranged in rows and columns and fixed to an active substrate 4, made for example of silicon in CMOS technology (conveniently used also for providing control electronics for the storage device). The array of transducers is arranged above a storage medium 5 (for example, made of polymeric material, ferroelectric material, phase-change material, etc.), and is relatively mobile with respect thereto. Each transducer 3 comprises a supporting element 6 made of semiconductor material, suspended in cantilever fashion above the storage medium 5, and an interaction element 7 (or tip), facing the storage medium 5, and carried by the supporting element 6 at a free end thereof. The interaction element 7 is able to interact locally with a portion of the storage medium 5, for writing, reading, or erasing individual information bits.
The relative movement between the storage medium 5 and the array of transducers is generated by a micromotor 10 coupled to the storage medium 5. The micromotor 10 is of a linear electrostatic type, made with semiconductor technologies, and operates on capacitive variations.
In detail, the electrostatic micromotor 10 comprises a stator substrate 12, and a rotor substrate 13 arranged in use above the stator substrate 12 (the term “rotor” is used herein, as usually occurs in this technical field, to indicate a mobile element without necessarily referring to a rotary movement). Typically, both the rotor substrate 13 and the stator substrate 12 are made of semiconductor material, for example, silicon.
The rotor substrate 13 is suspended above the stator substrate 12 by means of elastic elements (not illustrated herein), and has, at a facing surface 13a facing the stator substrate 12, a plurality of rotor indentations 14; the rotor indentations 14 are obtained, for example, by anisotropic chemical etching and extend towards the inside of the rotor substrate 13. The rotor indentations 14 are set at a regular distance apart from one another by a first pitch P1 in a sliding direction x. The rotor indentations 14 define between them rotor projections 15, extending towards the stator substrate 12.
The stator substrate 12 has, on a respective facing surface 12a facing the rotor substrate 13, an insulation layer 16, made, for example, of silicon oxide, on top of which a plurality of stator electrodes 17 is provided. The stator electrodes 17 are arranged at a regular distance apart from one another by a second pitch P2 in the sliding direction x. The second pitch P2 is different from, for example smaller than, the first pitch P1, and the stator electrodes 17 are staggered with respect to the rotor projections 15 in the sliding direction x.
Each pair constituted by one rotor projection 15 and by the underlying stator electrode 17 forms a plane parallel plate capacitor with misaligned plates. When a voltage is applied between the misaligned plates, a force is generated, which tends to bring them back into an aligned position. Consequently, by appropriately biasing the stator electrodes 17 (with the rotor substrate 13 set generally at a reference potential) with biasing voltages conveniently out-of-phase with respect to one another, it is possible to generate an electrostatic interaction force, which brings about a relative linear movement of the rotor substrate 13 with respect to the stator substrate 12 in the sliding direction x. In particular, due to the presence of the rotor indentations 14, the capacitance C of the aforesaid capacitor is variable with the relative displacement between the stator substrate 12 and the rotor substrate 13, and in particular is maximum when the stator electrode 17 is aligned with one of the rotor projections 15, and minimum when the stator electrode 17 is aligned with one of the rotor indentations 14. The electrostatic interaction force, which causes the relative movement of the rotor substrate 13 with respect to the stator substrate 12, is proportional to the resultant capacitive variation in the sliding direction x (in particular to the derivative of this variation).
The storage medium 5 is set on an external surface 13b of the rotor substrate 13, opposite to the facing surface 13a that faces the stator substrate 12. In this way, actuation of the electrostatic micromotor 10 causes a corresponding movement of the storage medium 5 in the sliding direction x, and a relative displacement thereof with respect to the transducers 3. In particular, by appropriately driving the electrostatic micromotor 10, it is possible to control positioning of the transducers 3 at desired points of the storage medium 5, where it is desired to carry out the operations of reading, writing, or erasure of the stored data.
As is shown schematically in FIG. 2, a control servomechanism is generally associated to the electrostatic micromotor 10; the control servomechanism comprises a position-sensing structure 18 designed to detect the relative position of the rotor substrate 13 with respect to the stator substrate 12, and a control unit 19, designed to carry out a feedback control of the actuation of the electrostatic micromotor 10 (and of the consequent positioning of the transducers 3), on the basis of the aforesaid detection of position. An electronic circuitry 20 is moreover connected to the array 2 for addressing the various transducers 3 (for example, via row and column multiplexers), and hence carrying out appropriate operations on the data stored in the storage medium 5.
In detail, and as shown in FIG. 3, the position-sensing structure 18, of a capacitive type, includes a first electrode 21a and a second electrode 21b, which are arranged above the insulation layer 16, laterally with respect to the stator electrodes 17 in the sliding direction x, for example in an area corresponding to an end portion of the stator substrate 12, and are biased at different voltages; and a third electrode 21c, set on the facing surface 13a of the rotor substrate 13 facing the stator substrate 12, which is set, in a rest position, between the first electrode 21a and the second electrode 21b. The first and second electrodes 21a, 21b form, with the third electrode 21c, a first sensing capacitor C1 and a second sensing capacitor C2, respectively. The surface area between the first and second electrodes 21a, 21b, and the third electrode 21c varies as a function of the position of the rotor substrate 13 with respect to the stator substrate 12, during its displacement in the sliding direction x. From a differential reading of the capacitance value of the first and second sensing capacitors C1 and C2, it is possible to determine the direction and amount of the aforesaid displacement, and so the relative position of the rotor substrate 13 with respect to the stator substrate 12.
The sensing structure described is not, however, optimized, due to the presence of a parasitic capacitance (as regards the aforesaid detection of position), which is formed between the third electrode 21c and the stator substrate 12. This parasitic capacitance brings about a lower sensitivity of the sensing structure to the variations of position, thus reducing the capacitive variation of the first and second sensing capacitors C1, C2 due to the relative displacement of the rotor substrate 13.